1. Technical Field (Field of the Invention)
The present invention relates to electronic amplifiers. More particularly, the invention concerns an electronic amplifier that has fast DC settling for providing accurate amplification at low frequencies.
2. Description of Related Art
Power amplifiers are widely used electronic devices. Power amplifiers can be used, for example, as audio, servo (motor control), or instrumentation amplifiers. Ideally, a power amplifier produces an output signal at a load that is identical to an input signal, except that the power of the output signal is greater than the power of the input signal. (Pout=Vout*Iout) greater than (Pin=Vin*Iin). In many applications it is desirable to have a power amplifier that has a frequency response which is substantially flat down to DC, for amplifying signals that include very low frequencies (for example, frequencies below 20 Hz), or DC.
Amplifiers commonly utilize feedback to reduce errors caused, for example, by inaccurate components, component mismatches, and power supply issues. A prior art:design for an amplifier circuit 100 that utilizes feedback is illustrated in FIG. 1. The amplifier circuit 100 has an input 101 and an output 102, and includes an amplifier 103, a resistor 104, an operational amplifier 106, a capacitor 108, and an adder 109. The amplifier 103 has an input 110 and an output 111. The operational amplifier 106 has an inverting input 112, a noninverting input 113, and an output 114. The adder 109 has an inverting input 116, a noninverting input 118, and an output 120. The prior art amplifier circuit 100 utilizes feedback implemented with a DC servo loop that includes the resistor 104, the operational amplifier 106, and the capacitor 108. To prevent degradation of the low frequency response of the amplifier circuit 100, the frequency response of the servo loop must be considerably lower than the minimum frequency that is to be amplified by the amplifier circuit 100. In many applications, for example audio amplifiers and servo amplifiers, this characteristic of DC servo loop feedback amplifiers and the general inability of DC servo loop feedback amplifiers to be used to amplify DC signals, is a serious disadvantage. The amplifier circuit 100 has the further disadvantage of producing a DC offset at the output 102, in response to an AC input signal that has a DC component at the input 101.
Feed-forward error correction is another technique that can be utilized in amplifier circuits to reduce output errors. However, a feed-forward amplifier circuit typically includes one or more filters to improve the AC characteristics of the amplifier. Filters are typically implemented with a capacitor or with another frequency dependent circuit element. A disadvantage of including a filter in an amplifier circuit is that the filter generally will degrade the low frequency response of the amplifier.
For circuits that can be implemented in integrated circuits, accurate component matching can be utilized to reduce errors caused by component mismatches. However this technique is not available for power amplifiers, due to voltage and current limitations of integrated circuits, which prevent implementing power amplifiers on integrated circuits.
In summary, existing designs are inadequate for implementing a power amplifier that can accurately amplify low frequency signals.
An illustrative embodiment of the present invention concerns an amplifier circuit that provides accurate amplification over a range of frequencies including very low frequencies, and DC. The amplifier circuit includes an input and an output, a voltage-to-current converter, a feed-forward resistive element, a buffer, an input resistive element, a feedback resistive element, and an operational amplifier. The voltage-to-current converter has an output, and an input that is coupled to the input of the amplifier circuit. The feed-forward resistive element has a first end and a second end. The first end of the feed-forward resistive element is coupled to the output of the voltage-to-current converter. The buffer has an output that is coupled to the output of the amplifier circuit, and an input that is coupled to the first end of the feed-forward resistive element. The input resistive element has a first end and a second end. The first end of the input resistive element is coupled to the input of the amplifier circuit. The feedback resistive element has a first end and a second end, and the first end of the feedback resistive element is coupled to the output of the amplifier circuit. The operational amplifier has an inverting input, a noninverting input, and an output. The inverting input is coupled to the second end of the input resistive element and to the second end of the feedback resistive element. The noninverting input is coupled to ground, an,d the output of the operational amplifier is coupled to the second end of the feed-forward resistive element.
Another aspect of the invention is a method for amplifying a voltage. Illustrative examples of various aspects of the invention are described in the sections below.
The invention provides a number of advantages, including the ability to accurately-amplify electronic signals that include very low frequency and DC components. The invention also provides a number of other advantages and benefits, which should be apparent from the following description.